1. Field of the Invention
The present invention relates to plasma process devices, and more specifically, to a plasma process device capable of performing a processing such as deposition, etching and ashing to a large size, rectangular glass substrate using plasma.
2. Description of the Background Art
Conventional plasma process devices to perform deposition, etching and ashing using plasma are known. One of known methods of generating plasma in such a plasma process device is an electron cyclotron resonance plasma excitation method according to which plasma is excited using a microwave and a DC magnetic field. In the electron cyclotron resonance plasma excitation method, however, stable plasma results only if the pressure is set to a level of several mTorr or less at the time of generating plasma. In addition, since the electron temperature in plasma is high, the plasma formed using the electron cyclotron resonance plasma excitation method is not suitable for the process such as deposition as described above. In the electron cyclotron resonance plasma excitation method, a DC magnetic field must be applied, which necessitates the entire device to have a large size. As a result, the manufacturing cost of the plasma process device is disadvantageously high.
Meanwhile, there is a known method of exciting plasma using the surface wave mode of microwave propagating through dielectric rather than using an electron cyclotron resonance method with a DC magnetic field as described above. The plasma excitation method using the surface wave of a microwave can produce stable plasma if the pressure is set in a relatively broad range from several ten mTorr to several Torr or higher, Since the electron temperature in the plasma is relatively low, surface wave excited plasmas are suitable for any of the above processings such as deposition may result.
In a process such as plasma CVD (Chemical Vapor Deposition) and etching, a reaction gas must be introduced uniformly over the entire surface of substrate subject to a reactive process. This is to assure process condition uniformity for deposition, etching or the like over the entire substrate. As one known means for achieving this is the use of a shower plate to supply a reaction gas in a plasma process device. Herein, the shower plate refers to a plate shaped member positioned to oppose a substrate to be processed and having a plurality of reaction gas inlets to introduce a reaction gas into a processing chamber in which the substrate is placed.
As a conventional plasma process device using a method of exciting plasma using the surface wave of a microwave as described above together with a shower plate, a plasma process device using a radial line slot antenna has been known. FIG. 16 is a schematic cross sectional view of a conventional plasma process device using a radial line slot antenna. Referring to FIG. 16, the plasma process device will be described.
Referring to FIG. 16, plasma process device 150 includes a vacuum vessel 156 as a processing chamber, a shower plate 153, a dielectric plate 152, a radial line slot antenna 151 and an exhaust pump 155. In vacuum vessel 156, a circular substrate 154 subjected to deposition process or the like is placed on a substrate holder. Shower plate 153 of dielectric is provided on the upper wall surface of vacuum vessel 156 opposing substrate 154. Dielectric plate 152 is provided above shower plate 153 with a gap 163 therebetween. Radial line slot antenna 151 is provided on dielectric plate 152. Shower plate 153, dielectric plate 152 and radial line slot antenna 151 have a circular shape when viewed from the top. A reaction gas inlet passage 157 is formed to connect the gap 163 between shower plate 153 and dielectric plate 152. A reaction gas introduced to gap 163 from reaction gas inlet passage 157 is let into vacuum vessel 156 through the gas inlets formed in shower plate 153.
Substantially homogeneous plasma 158 is formed over the entire surface of substrate 154 from the reaction gas by the microwave introduced into vacuum vessel 156 from radial line slot antenna 151 through dielectric plate 152, gap 163 and shower plate 153 formed of dielectric. With plasma 158, a processing such as deposition may be performed on the surface of substrate 154. The reaction gas which have not contributed to the processing and the gas generated by the reaction at the substrate surface are let out of vacuum vessel 156 through exhaust pump 155.
FIG. 17 is a perspective cross sectional view of the radial line slot antenna shown in FIG. 16. Referring to FIG. 17, the radial line slot antenna will be described.
Referring to FIG. 17, radial line slot antenna 151 includes a coaxial waveguide 160, a ground plate 159 formed of conductor, a dielectric plate 161 and a slot plate 164 of conductor having slots 162. Dielectric plate 161 is provided under ground plate 159. A slot plate 164 is provided under dielectric plate 161. Coaxial waveguide 160 is connected to dielectric plate 161. A microwave is transmitted to dielectric plate 161 from coaxial waveguide 160. Dielectric plate 161 serves as a radial microwave transmission path. A microwave is radiated through slots 162 formed in slot plate 164 from the entire bottom surface of radial line slot antenna 151.
In the conventional plasma process device using the radial line slot antenna, plasma excitation with a microwave and uniform supply of a reaction gas to the processing chamber using the shower plate are simultaneously performed. The plasma process device using the radial line slot antenna described above suffers from the following problem.
More specifically, referring to FIG. 16, in the conventional plasma process device, a microwave used to form plasma 158 is supplied from radial line slot antenna 151 into vacuum vessel 156 as a processing chamber through dielectric plate 152, gap 163 and shower plate 153. At this time, gap 163 serving as a transmission path for the microwave also function as a supply passage for a reaction gas to vacuum vessel 156. As a result, there is the reaction gas to generate plasma in gap 163. Therefore, the microwave transmitted from radial line slot antenna 151 into vacuum vessel 156 can generate plasma when the gas pressure in gap 163 and the microwave conditions are inappropriate. If plasma is thus generated in gap 163, shower plate 153 and dielectric plate 152 could be damaged by this plasma. In order to prevent the plasma (abnormal plasma) from being generated in gap 163, the pressure of the reaction gas in gap 163 was set significantly higher than the pressure of the reaction gas in vacuum vessel 156. This is for the following reason: electrons in the reaction gas are accelerated by an electric field by the microwave. If however the pressure of the reaction gas in gap 163 is set to a high level of 10 Torr or more, for example, the electrons can collide with other gas atoms or molecules before they are accelerated by the above electric field. As a result, the electrons will no longer have enough energy to generate plasma, so that the plasma can be restrained from being generated in gap 163.
While the pressure of the reaction gas in gap 163 is set to a high level, the pressure inside vacuum vessel 156 must be maintained at a level of several mTorr. As a result, the pressure of the reaction gas in gap 163 is kept at a high level, while the supply of the reaction gas to vacuum vessel 156 must be sufficiently small. Therefore, the easiness for the reaction gas to flow (conductance) through the reaction gas inlets formed in shower plate 153 must be small. In order to realize such small conductance, fine gas inlets in shower plate 153 must be formed with extremely high precision (a precision in the order of 10 .mu.m). Meanwhile, shower plate 153 must be formed using dielectric such as ceramic to allow a microwave to propagate. It is extremely difficult to form gas inlets having such high precision in the dielectric. As a result the manufacturing cost of the shower plate is disadvantageously high.
Since the pressure of the reaction gas in gap 163 must be kept at a high level, process conditions such as the component ratio or flow rate of the reaction gas can be hardly precisely controlled. As a result, the process conditions such as the gas component ratio are shifted from a prescribed numerical range, which makes it difficult to adjust the process conditions, and plasma process such as deposition can no longer performed in a prescribed condition.
In addition, as shown in FIGS. 16 and 17, the conventional radial line slot antenna 151 is circular, in order to apply it to a rectangular substrate for used in a TFT liquid crystal display device or the like, shower plate 153 larger than the rectangular substrate must be used so that the entire surface of the rectangular substrate can be covered. Such rectangular substrates have been increased in size from 500 mm.times.500 mm to 1 m.times.1 m as the liquid crystal display device has come to have a larger size. Radial line slot antenna 151 and shower plate 153 are formed using dielectric such as ceramic as described above. Since it would be difficult to form a large size dielectric plate of ceramic or the like, the conventional plasma process device cannot cope with the large size rectangular type substrate.